Phase In Heat-affected Zone Research Articles

Formation mechanism of massive phase in the heat affected zone of Ti-6Al-4V fabricated by forging-additive hybrid manufacturing

The heat affected zone (HAZ) of Ti-6Al-4V forging-additive hybrid manufacturing parts is always featured with large-sized massive phase (αm) embedded in the previous equiaxed α region. However, the formation mechanism of such a unique microstructure remains unclear. To reveal the formation mechanism of massive phase in HAZ, we investigated both the microstructures of HAZs of single-pass and multi-pass samples, and also carried out a finite element (FE) simulation to obtain the temperature-field evolution of the whole process. Our results show that, in the single-pass sample, patchy αm forms in previous equiaxed α region of HAZ, while needle-like martensite α' forms in previous lamellar α regions close to the equiaxed α region. Electron probe microanalysis (EPMA) results further show that, the high temperature β phase transformed from previous equiaxed α can inherit the initial composition due to the large size of previous equiaxed α and the insufficient diffusion during rapid heating, while the β phase transformed from previous lamellar α possesses the bulk average composition due to the small distance between neighbor lamella. It is this difference in composition that resulting in the formation of αm and α' in the previous equiaxed α region and the previous lamellar α region, respectively, during subsequent cooling. In the following thermal cycles of multi-pass deposition, some αm grains remain during the next heat cycle and finally evolve into a few large-sized αm within the previous equiaxed α region, as observed in the multi-pass sample. These findings are helpful for understanding and controlling the microstructural evolution of HAZ in hybrid manufactured Ti alloys.

Microstructure and wear behavior of IC10 directionally solidified superalloy repaired by directed energy deposition

• The microstructure and wear behavior of different zones of IC10 alloy repaired by directed energy deposition are studied. • The size of primary γ′ phase in HAZ is smaller than that in the substrate, and the size and fraction of γ′ phase are decreased along building direction. • The dominant wear mechanism of the repaired alloy is abrasive wear, and the wear resistance is closely related to the fraction of γ′ phase. Directed energy deposition has been used to repair superalloy components in aero engines and gas turbines. However, the microstructure and properties are generally inhomogeneous in components because of the different processing histories. Here, the microstructures and wear behavior of different zones (substrate, HAZ, and deposit) are investigated for the IC10 directionally solidified superalloy repaired by the directed energy deposition process. It is found that the microstructure of the deposited layers is strongly textured with a <001>-fiber texture in the building direction, and the texture intensity is continuously increased along the building direction. Two kinds of γ′ phase (primary and secondary γ′ phase) can be found in the heat-affected zone (HAZ), and the average size of primary γ′ phase is smaller than that in the substrate due to liquation. In the deposit layers, the size of γ′ phase is much smaller than those in the substrate and the primary γ′ phase of HAZ; both size and the fraction of the γ′ phase decreases with the increase of building height. The wear rate of the substrate is the smallest, indicating the best wear resistance; while the wear rate of HAZ is the largest, indicating the worst wear resistance in the repaired sample. The wear rates in the deposit layers increase from the bottom to the top zones, showing a decreasing wear resistance. Abrasive wear is found to be the dominant wear mechanism of the repaired alloy, and the resistance to which is closely related to the fraction of γ′ phase in the microstructure. The understanding of the influence of microstructure on wear resistance allows for a more informed application of inhomogeneous superalloy components repaired by directed energy deposition in industry.

Microstructure and impact mechanical properties of multi-layer and multi-pass TIG welded joints of Al−Zn−Mg alloy plates

Abstract The microstructure and mechanical properties of multi-layer multi-pass TIG welded joints of Al−Zn−Mg alloy plates were studied. The phase constituent and microstructure of different regions of the welded joints were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM) and energy disperse spectrum (EDS), while the mechanical properties were evaluated according to the impact test. A dispersively distributed spherical and needle-like η(MgZn2) phase was obtained in the welding seam. The phase composition of the heat-affected zone (HAZ) was α(Al)+η(MgZn2)+Al6Mn, and there were a large number of dispersively precipitated nanoscale particles. The welded joint zone had the highest impact toughness as compared with the other parts of the joint. The MgZn2 phase in the weld zone contributed to the improved toughness of the joint. Al2MgCu phase in HAZ was proven to act as a crack source during fracture.

Effect of Microstructure on the Corrosion Resistance of TIG Welded 1579 Alloy.

The paper studies microstructure, chemical composition and corrosion activity of the tungsten inert gas welded joint of the Al-Mg-Sc alloy. An intensive corrosion attack of the heat affected zone (HAZ) was found due to precipitation of secondary phases at recrystallized grain boundaries. The ccorrosion process initiated along the boundary of α-Al grains, where a high concentration of anodic (Mg2Si and Mg2Al3) and cathodic phases ((MnFe)Al6) was observed. Increased temperatures during welding led to coalescence of the anodic phases in HAZ. Additionally, HAZ was found to be enriched with hard intermetallic compounds (Mg2Si and (MnFe)Al6). This area had a higher microhardness (930 MPa) compared to base metal (BM, 895 MPa) and fusion zone (FZ, 810 MPa). The volume fraction of secondary phases was 26% in BM, 28% in FZ and 38% in HAZ. The average grain size increased in the following order: (9 ± 3) µm (BM) < (16 ± 3) µm (HAZ) < (21 ± 5) µm (FZ). A plasma electrolytic oxidation (PEO) coating of aluminum-based material was applied to protect the weld from oxidation. The PEO-coating provided a high corrosion protection in the aggressive Cl−-containing environment.

Study of local-zone microstructure, strength and fracture toughness of hybrid laser-metal-inert-gas-welded A7N01 aluminum alloy joint

Mechanical properties of hybrid laser-metal-inert-gas-welded A7N01-T5 aluminum alloy joints were studied by using local samples that were extracted from the base metal (BM), heat-affected zone (HAZ), and fusion zone (FZ) of the joint to investigate the triangular relationship of microstructure, strength and fracture toughness of the local zones. The BM had the highest yield strength, ultimate tensile strength (UTS) and lowest elongation, which contrasts with the FZ. The yield strength of the HAZ is lower than that of the BM, whereas its UTS is very close to that of the BM, and its elongation is higher than that of the BM. The fracture toughness of the three local zones decreased as HAZ>BM>FZ. To analyze differences in local mechanical behavior, the detailed microstructure of the three local zones was studied by optical microscopy and electron backscattered diffraction, whereas the fracture surface and precipitation were studied by scanning and transmission electron microscopy. The variation of grain size, especially the morphology and distribution of strengthening phase in HAZ in welding process is the key factor that leads to its different mechanical properties from that of BM, which can be elucidated by different dislocation mechanism, sheared mechanism or Orowan mechanism. The as-cast microstructure and second-phase particles that segregate between dendritic branches provide the FZ with the lowest yield strength and UTS. The factors including area fraction of the precipitates, the difference of strength between the matrix and the grain boundaries, the precipitate-free zone along grain boundaries, as well as the grain boundaries angle are taken into account to explain the difference of fracture toughness among BM, HAZ and FZ, and their fracture modes.

Liquation of various phases in HAZ during welding of cast Inconel* 738LC

The weld heat affected zone (HAZ) of tungsten inert gas (TIG) welded cast IN 738 superalloy has been examined. HAZ microfissuring was observed in alloy welded in solution treated and overaged conditions. Closer and careful microstructural examination by analytical scanning electron microscopy revealed the liquation of boride, sulphocarbide, MC type carbides, γ′ precipitate particles and γ-γ′ eutectic, which were present in the preweld alloy, and the formation of resolidified constituents along the microfissured HAZ grain boundaries, thereby indicating that HAZ cracking in this alloy involves liquation cracking. Liquation of these phases, as well as characteristics of the intergranular liquid film contributing to the low resistance to HAZ cracking of the alloy have been examined and are discussed.